1. Technical Field of the Invention
The present invention relates to frequency transposition and applies advantageously to, but is not limited to, the field of radiofrequency communications, for example mobile telephone applications, in which the radiofrequency circuits make extensive use of frequency transposition devices, or frequency mixers, both in transmission and reception.
2. Description of Related Art
In transmission, the purpose of the frequency mixers, which in this case are frequency upshift circuits, is to transpose the information from the baseband to around the transmission carrier.
FIG. 1 illustrates schematically the usual structure used for frequency transposition devices of the prior art.
In the top part of FIG. 1, the reference MIX denotes a frequency transposition means, or mixer (here a frequency upshifter), having an input terminal BES for receiving an incident signal SI, for example in the baseband or at an intermediate frequency. The mixer MIX also has a control terminal BCO for receiving a local oscillator signal LO, for example around 2 GHz in a mobile telephone application using CDMA (Code Division Multiple Access) systems. The mixer MIX further includes an output terminal BS for delivering the output signal STR that is a transposed signal whose frequency spectrum is situated around the fundamental frequency of the local oscillator signal and odd number rank harmonics. The amplitude of these harmonics decreases proportionally to their rank number, in other words they decrease, in decibels, with a slope of −20 dB per decade.
Thus, in practice, the mixer MIX is followed by a bandpass filter centered around the fundamental frequency of the local oscillator, such that only the part of the spectrum centered around this fundamental frequency is conserved.
The usual structure used for these mixers is a generally differential structure of the Gilbert-cell-type such as is illustrated schematically in the middle part of FIG. 1.
Such a cell is well known to those skilled in the art and only its essential features are recalled here.
More precisely, such a cell comprises a differential transducer block BTC for converting the input signal (voltage) present across the terminals BES into a differential current. Here, this block BTC comprises a stage formed by a differential pair of transistors whose respective bases are connected to the input terminals via two capacitors. The collectors of the two transistors of this stage form the output terminals of this transducer block BTC. The block BTC can, of course, comprise several stages.
The transistors of the stage of the block BTC are biased by conventional biasing means MPL notably comprising resistors together with a voltage source.
A current switching block BCC is connected to the output of the transducer block BTC, in other words to the collectors of the transistors of this block BTC. The switching block BCC alternately routes the current towards one or the other of the two output terminals BS at the frequency of the local oscillator signal LO received at the terminals BCO. This block BCC conventionally comprises two pairs of transistors.
Each resistor ZL, connected between the output terminals BS of the block BCC and the power supply Vcc, represents the output load of the mixer MIX.
The block BTC converts the power or the voltage applied to the input BES into a differential current that is an image, assumed to be linear, of the input signal. This linear signal is then chopped by a non-linear square function (+1, −1, +1, −1 . . . ), produced by the two-way switch BCC, at the frequency of the signal LO, this two-way switch acting as a dynamic current router. The output signal is acquired across the terminals of the differential load 2ZL.
In other words, as also illustrated schematically in the lower part of FIG. 1, the signal STR present at the output terminal BS of the mixer MIX corresponds to the incident signal SI multiplied by +1 (or non-inverted) and then −1 (or inverted) at the rhythm of the local oscillator periodic signal LO, generally amplitude limited at +1 and −1.
Thus, in such a conventional mixer, the control input, or local oscillator input, receives a periodic transposition signal (local oscillator signal) having the desired transposition frequency and with a fixed level of power necessary to drive the transistors of the switching block BCC of the Gilbert cell.
Moreover, the power of the transposed signal is, neglecting losses, equal to the power of the incident signal.
Under these conditions, and with the assumption that a low power incident signal is available which needs to be transmitted, after transposition, with a high power, it is then necessary to carry out a signal amplification which generally comprises an amplification of the incident signal before transposition and an amplification after transposition.
However, where high amplification of the incident signal is necessary in some applications, for example in mobile telephones, this turns out to be particularly difficult to achieve since the linearity of the incident signal needs to be preserved during the amplification so as not to lose information during the mixing (transposition) process.
A need accordingly exists to provide a solution to this problem.